The present invention is directed to a light quantity-control device such as a shutter or a beam limiting device used in an image pickup device of a video camera, a still camera or the like, or a projector, and other various optical apparatuses. Also, it relates to a shutter device which blocks a photographed light quantity or projected light quantity with, e.g., a blade member assembled in a barrel of an optical lens, or a light quantity-control device which makes larger or smaller a photographed light quantity or projected light quantity thereby to control it, and an electromagnetically-driving device which opens or closes the blade member.
In general, light quantity-control devices contained in these optical apparatuses, such as a shutter device and a beam limiting device, have been widely known as a shutter device or beam limiting device having a lens barrel, a base (bottom board) assembled in the lens barrel, and a thin plate-type blade member openably attached to the base, in which a light quantity is blocked or controlled by opening or closing the blade member.
Such a blade member is composed of one or more parts, pivotably or slidably supported about an optical-axis aperture of the base by a pivot such as a pin, and further arranged so that it can be driven by an electromagnetically-driving device to be opened or closed. The electromagnetically-driving device includes a rotor with a permanent magnet, and an excitation coil for providing a rotation force to the rotor. Recently, as for particularly these light quantity-control devices, ones which are compact, lightweight, and small in power consumption have been required as downsizing and weight reduction of optical apparatuses have occurred.
According to a conventional structure widely adopted for a driving device like this, a rotor is configured by rotatably supporting a cylindrical permanent magnet inside a coil frame with an external circumference to be wound with a coil, the coil is wound around the external circumference of the coil frame, and the external circumference of the coil is magnetically shielded by a yoke. A device having a configuration like this has a problem of upsizing, and particularly has a problem such that the outer diameter of a core part becomes larger, which has an external circumference with a coil wound around it, a magnet rotor placed therein, and a rotating shaft of the rotor placed inside the rotor.
Hence, e.g., JP-A-2001-298936 and JP-A-2002-049076 have proposed a method including the steps of: arranging a magnet rotor formed in a hollow, cylindrical shape and an excitation coil annularly wound along a direction of the axis above and below; leading a magnetic field of the excitation coil to the circumference of the magnet rotor using the yoke made a soft magnetic material thereby to form magnetic poles. In the devices proposed in the patent documents, a magnet rotor is formed in a hollow, cylindrical shape, an inside yoke and an outside yoke are disposed in a central aperture portion of the rotor and an external circumference portion of the rotor so as to surround the magnet of the rotor, respectively. The inside and outside yokes lead a magnetic force from a coil disposed in a different location from the magnet rotor to the circumference of the magnet rotor, whereby a rotation force is generated.
An electromagnetically-driving device like this can be made more compact by reduction in its outer diameter because of having a structure such that a hollow magnet rotor is disposed inside an outside yoke in a thin-plate cylinder shape, and an inside yoke in a rod shape is provided inside the magnet rotor. In addition, the magnet rotor obtains a rotation torque in an external circumference portion close to the permissible device outer diameter and as such, a larger rotation force can be generated even when the device is compact. Further, a magnetic circuit constituted by the outside yoke, magnet rotor and inside yoke in order has the following features: the permeance of the whole magnetic circuit is high; and electric power consumption can be reduced. This is because the gap between the outside yoke and magnet rotor and the gap between the magnet rotor and inside yoke can be reduced in size to the minimum as long as rotation of the rotor is permissible.
For an electromagnetically-driving device having such structure, the following structure is adopted in order to rotatably support the magnet rotor. In the device described in JP-A-2001-298936, the inside yoke to be disposed in a center portion of the hollow, cylindrical permanent magnet is formed in a hollow shape, the rotating shaft of the magnet rotor is put into the center hole of the inside yoke, and both the ends of the rotating shaft are borne by the board and the outside yoke.
In the device described in JP-A-2002-049076, a columnar inside yoke is disposed in a center hole of the hollow, cylindrical permanent magnet, and a concave groove for use of bearing is formed in an end face located in a leading end of the inside yoke. The two opposite ends of the rotating shaft provided in a center portion of the magnet rotor are borne by the concave groove formed in the end face of the inside yoke and a bearing hole formed in the base.
According to the bearing structure disclosed in JP-A-2001-298936, the inside yoke is provided inside the magnet rotor, and the rotating shaft of the rotor is provided further inside the inside yoke. Therefore, the device has raised problems concerning downsizing of the device as follows. The first is that the device has a relatively larger diameter and is of large size. The second is that it is difficult to align the annularly formed inside yoke with the rotating shaft formed in the center of the rotor. The third is that the magnetic gap between the magnet and the inside yoke is changed by rotation of the rotor.
According to the bearing structure proposed in JP-A-2002-049076, the concave groove for use of bearing is formed in the shaft end face of the columnar inside yoke. Thus, a wedge-shaped notch is formed in the end face of leading end of the inside yoke, whereby a magnetic force line formed in a loop between the inside yoke and outside yoke is attenuated. In other words, the bearing structure has the following disadvantage: the concave groove remarkably lowers the permeance of the magnetic circuit, which causes a problem on the magnetic circuit formed between the outside yoke and inside yoke. Even with any of the bearing structures proposed in JP-A-2001-298936 and JP-A-2002-049076, reduction in shaft diameter of the rotor makes the processing of the bearing portion more difficult and causes a problem in durability against friction.
Therefore, it is an object of the invention to provide an electromagnetically-driving device, by which, in supporting a cylindrical hollow magnet rotor by an inside yoke, magnetic gaps formed between the magnet rotor and inside and outside yokes respectively can be maintained precisely and with stability, and a large output can be obtained without reduction in permeance of the magnetic circuit even when the device is small. The invention seeks to accomplish downsizing of electromagnetically-driving devices. Therefore, it is another object of the invention to provide a light quantity-control device which is small and compact, and particularly small in device outer diameter.
Further objects and advantages of the invention will be apparent from the following description of the invention.